1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an external optical modulator. Although the present invention is suitable for a wide scope of application, it is particularly suitable for converting N number (wherein N is positive even integer greater than 2) of electrical signal channels into N/2 independent optical channels by using N/2 modulators.
2. Discussion of the Related Art
An external optical modulator (EOM) is an important component in wavelength division multiplexed (WDM) optical transmission systems. In such systems, an electrical signal representing the information to be transmitted is applied to the EOM, which then modulates a continuous wave (CW) laser beam that propagates the modulated light through the transmission system. One of the advantages of external optical modulation over directly modulating the laser is that data can be transmitted relatively chirp-free, thereby reducing errors in long distance transmission. Chirp is an instantaneous change in optical frequency, which accompanies the process of directly modulating the laser diode. Chirp interacts with the dispersion profile of the transmission fiber to severely limit the distance over which error-free data transmission is possible. External optical modulators can be designed with little or no chirp, enabling a much higher transmission distance than can be obtained with direct modulation.
In a conventional optical modulator, the dual electrode design concept was developed so that the data and its complement can be separately applied to the two electrodes of the dual electrode modulator. This is also known as differential driving. By adjusting the relative amplitudes of the data and its complement, the chirp parameter of the modulator can be fixed to any desired value between xe2x88x92xe2x88x9d and +xe2x88x9d.
Conventionally, externally modulated WDM transmission systems have employed one EOM for each electrical input signal. Further, the external optical modulator is one of the more expensive optical components employed in an optical system. It would therefore be advantageous to provide an apparatus wherein two or more electrical input signals are used to modulate two optical channels with one external optical modulator.
Accordingly, the present invention is directed to an external modulator that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide an apparatus that modulates two optical channels with four independent subcarrier channels on two modulators and has four modulated optical output channels.
Another object of the present invention is to provide an apparatus that modulates three optical channels with six independent subcarrier channels on three modulators and has six modulated optical output channels.
A further object of the present invention is to provide an economically more feasible apparatus for modulating more than one optical channel in a single modulator.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the present invention includes two Mach-Zehnder interferometer-type modulators for externally modulating two independent optical input signals with first, second, third and fourth electrical input signals, wherein a first modulator includes a first electrode receiving the first electrical input signal, a second electrode receiving the second electrical input signal, a first optical signal path co-propagating the first optical input signal with the first electrical signal and counter-propagating the second optical input signal to generate a first modulated optical signal, and a second optical signal path co-propagating the second optical input signal with the second electrical input signal and counter-propagating the first optical input signal to generate a second modulated optical signal, and wherein a second modulator includes a third electrode receiving the third electrical input signal, a fourth electrode receiving the fourth electrical input signal, a third optical signal path co-propagating the first optical input signal with the third electrical input signal and counter-propagating the second optical input signal to generate a third modulated optical signal, and a fourth optical signal path co-propagating the second optical input signal with the fourth electrical input signal and counter-propagating the first optical input signal to generate a fourth modulated optical signal.
In another aspect, the present invention includes three Mach-Zehnder interferometer-type modulators for externally modulating three independent optical input channels with first, second, third, fourth, fifth and sixth electrical input signals, wherein a first modulator includes a first electrode receiving the first electrical input signal, a second electrode receiving the second electrical input signal, a first optical signal path co-propagating the first optical input signal with the first electrical input signal and counter-propagating the second optical input signal to generate a first modulated optical signal, and a second optical signal path co-propagating the second optical input signal with the second electrical input signal and counter-propagating the first optical input signal to generate a second modulated optical signal, wherein a second modulator includes a third electrode receiving the third electrical input signal, a fourth electrode receiving the fourth electrical input signal, a third optical signal path co-propagating the third optical input signal with the third electrical input signal and counter-propagating the second optical input signal to generate a third modulated optical signal, and a fourth optical signal path co-propagating the second optical input signal with the fourth electrical input signal and counter-propagating the third optical input signal to generate a fourth modulated optical signal, and wherein a third modulator includes a fifth electrode receiving the fifth electrical input signal, a sixth electrode receiving the sixth electrical input signal, a fifth optical signal path co-propagating the third optical input signal with the fifth electrical input signal and counter-propagating the first optical input signal to generate a fifth modulated optical signal, and a sixth optical signal path co-propagating the first optical input signal with the sixth electrical input signal and counter-propagating the third optical input signal to generate a sixth modulated optical signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.